During the replacement of the ballcock assembly in a toilet flush tank for repairing a flush tank, the flush tank is first emptied of the flushing water. Due to the structure of the flush valve assembly which is mounted on the bottom of the tank, residual water remains in the tank after the tank has been emptied of the flushing water. Thus, when the ballcock assembly is removed, the residual water would normally leak from the bottom of the flush tank through the opening where the ballcock assembly is mounted. This results in wetting the floor with considerable water and the necessity to clean up the water after the assembly is replaced.